The present invention relates to cathode ray tube (CRT) devices such as video display terminals or television sets, and more particularly to reducing electro-magnetic emissions from horizontal sweep output sections of such CRT devices.
Most CRTs utilize a rapidly varying magnetic field created by specially wound coils to sweep a ray of electrons from a heated cathode across a phosphor coated screen to form words and/or images. The coil that controls the horizontal motion of the cathode ray is called the horizontal deflection circuit, and it is driven by the horizontal sweep circuit. Although there are some variations in timing, the cathode ray typically sweeps horizontally across the screen in approximately 1/25,000 th of a second.
During each horizontal sweep, the horizontal sweep circuit drives the horizontal deflection coil with a ramp or sawtooth signal that controls the horizontal sweep of the cathode ray across the CRT screen. At the end of each horizontal sweep, an extremely rapid ramp signal causes the cathode ray to sweep back to the starting point to begin the next horizontal sweep. Because of the extremely rapid trip back to the starting point, the transformer that drives and controls the sweep and rapid return sweep is called a flyback transformer. Thus, the flyback transformer is driven by a ramp of current that is approximately 1/25,000 th of a second in duration, followed by an oppositely sloped current ramp that is extremely short in duration.
With a sweep cycle of 1/25,000 of a second, it is understandable that the flyback transformer operates at approximately 25,000 hz, and because the flyback transformer is driven by a ramp shaped current, it is understandable that the magnetic flux of the flyback transformer is rich in harmonics of 25,000 hz.
In order to control the width of the horizontal sweep across the screen, many video display terminals and/or television sets employ a variable coil that is in series with the primary winding of the flyback transformer, which is called the horizontal width coil. Those horizontal sweep circuits which have a horizontal width coil, usually have a non-adjustable horizontal linearity coil in series with the horizontal width coil. The horizontal linearity coil has a ferrite core which is magnetized such that its inductance is a function of both the level and the direction of the current passing through it. The horizontal linearity coil compensates for the fact that the path of the cathode ray as it sweeps across the screen has non-linearities. A non-linear sweep would mean that some characters or images would have undesirable uneven proportions from left to right on the screen. Thus, the output of the horizontal sweep circuit has a linearity coil, a width coil and a flyback transformer primary.
The horizontal width coil and the horizontal linearity coils are basically solenoids in shape. If these coils were not in the proximity of other conductive or permeable material, each would exhibit a toroidal magnetic flux field. Further, because the flyback transformer is a non-ideal inductive transformer, it has a leakage flux. Because of the horizontal sweep drive current flowing through these inductive components, their individual fluxes will have harmonics in the very low frequency (VLF) band of electro-magnetic emissions.
A problem with the above described horizontal sweep output circuit has arisen because of its level of electro-magnetic emissions. Many countries, especially European countries, have decided to control the amount of ambient electro-magnetic emissions in the workplace. To this end, these countries have specified a maximum level of VLF electro-magnetic emissions that are permitted in the near field proximity of a video display or similar device. Sweden, for example, specifies that video displays shall have magnetic emissions that have a magnetic induction that is less than 24 milli-Teslas per second and a magnetic flux density less than 50 nano-Teslas at a distance of 0.3 m. from the front surface of the display. Additionally, Sweden specifies that at 0.5 m. from any exterior surface that a video display shall have a magnetic induction of less than 24 milli-Teslas per second and a magnetic flux density of less than 50 nano-Teslas. Because of the level of net electro-magnetic emissions from the horizontal width coil, the horizontal linearity coil and the flyback transformer, some video displays are unable to meet the lower European specifications, such as Sweden's.